The present invention relates to an ignition control system for an internal combustion engine adapted to vary the maximum closed-circuit rate characteristic of an ignition coil according to the engine speed or the power source voltage.
An induction discharge type igniter used for an internal combustion engine flows a current in the primary side winding of an ignition coil, and generates a high voltage energy in the secondary side winding of the ignition coil by interrupting the current, to thereby discharge a spark ignition plug connected to the secondary side winding of the ignition coil.
The high voltage energy in the secondary side winding of the ignition coil generally has a relation to a current (hereunder referred to as "an interrupting current") when the primary side winding of the ignition coil is broken. Therefore, in order to produce an energy necessary to ignite an internal combustion engine, it is necessary to energize the primary side winding of the ignition coil until the interrupting current reaches a value necessary to ignite.
The energizing time of the ignition coil until the interrupting current reaches a predetermined value depends upon the battery voltage, the primary side inductance of the ignition coil and the primary side resistance of the ignition coil.
The rate of the ignition coil energizing time with respect to an ignition period (hereunder referred to as "a closed-circuit rate") alters depending upon the engine speed.
Further, in order to ensure a time required for a spark discharge, it is necessary to set the maximum value of the closed-circuit rate (the maximum closed-circuit rate) if the closed-circuit rate is large. Therefore, it is necessary to control the energizing time of the ignition coil so as to produce a desired interrupting current against the above-described variation and to also ensure a discharge time by setting the maximum closed-circuit rate.
An ignition control system for controlling the energizing timing of an ignition coil so that the period that the primary current of the ignition coil reaches a predetermined value becomes a predetermined rate of the ignition period of an internal combustion engine and that a constant closed-circuit rate is obtained at the large closed-circuit rate time has been proposed as disclosed, for example, in Japanese Patent Laid-Open No. 40141/1978 as a prior art capable of controlling the energizing time of the ignition coil as described above.
In the abovementioned ignition control system, it is necessary to attain a long energizing time of the ignition coil so as to produce an energy necessary to ignite in high speed rotation range of the engine in case that a battery voltage is low or the primary side inductance of the ignition coil is large, and the maximum closed-circuit rate is set to a relatively high value.
Since the closed-circuit rate reaches the maximum value in the intermediate speed rotation of the engine in the above ignition control system in this case, the conventional control system in which the closed-circuit rate is set to the large predetermined maximum value has such disadvantages that the igniter and the ignition coil are overheated.
If the battery voltage becomes high due to the above set of the closed-circuit rate, the control system has another disadvantage that the ignition energy generated in the high speed rotation of the engine becomes excessive for the requirement.